1. Field of the Invention
The present invention relates to a cathode structure of a magnetron for microwave ovens, industrial microwave heating, and medical use.
2. Description of the Prior Art
FIG. 1A shows a longitudinal sectional view of a tube main body of a magnetron used generally in microwave ovens and the like. In FIG. 1A, reference numeral 1 designates a cathode for emitting electrons, 2 designates an anode having cavity resonators formed therein, and there are provided with magnetic pole pieces 3, 4 for concentrating magnetic line of force applied from the outside of the tube main body into an operating space formed by the cathode 1 and the anode 2. Sealing cylinders 5, 6 abut respectively against opposite ends of a cylindrical portion of the anode 2 to enclose the magnetic poles 3, 4. Further, a ceramic member serving as a sealing member supports the cathode 1, and a stem ceramic 8 is joined with the sealing cylinder 5 together with a cathode terminal 7 which supplies a cathode current. And an exhaust pipe support 11 holding an exhaust pipe 10 enclosing an antenna 9 extending from a specific vane of the anode 2 is joined with an antenna ceramic 12 serving as a radiation window of the generated microwaves, and the antenna ceramic 12 is in turn joined with the sealing cylinder 6.
FIG. 1B is a longitudinal sectional view of the prior art cathode structure of the magnetron shown in FIG. 1A.
In FIG. 1B, a filament 22 of a spiral shape made of Th-W (thoriated tungsten) constituting the cathode 1 is supported and fixed at its opposite ends respectively to an upper end shield 23 and a lower end shield 24. The upper end shield 23 is secured to a center lead 26, and the lower end shield 24 is secured to a side lead 25, and these leads 25, 26 are respectively inserted into holes 27a, 27a of an insulating ceramic 27 to pass therethrough. Extending ends of the leads 25, 26 are secured air tightly to the cathode terminal 7 made of a metal fixed to a metallized portion formed at an outer end of the ceramic 27. The sealing cylinder 5 made of a metal has one end secured air tightly to the insulating ceramic 27, and the other end of the sealing cylinder 5 is fixed to the anode 2 (FIG. 1A) to mechanically hold the magnetron and to shield from the atmosphere. The reference numeral 28 designates a spacer made of ceramic, and FIGS. 1C and 1D are respectively a plane view and a longitudinal sectional view. In FIGS. 1C and 1D, the spacer 28 has two spacer holes 28a and 28b, and the side lead 25 is inserted into the spacer hole 28a and the center lead 26 is inserted into the spacer hold 28b to suppress the movements of both leads 25, 26 and to reinforce the securing of both leads 25, 26.
Such a cathode structure of a magnetron is disclosed, for example in JP-A-54-109364 (Japanese Patent Laid-Open Publication No. 54-109364).
Recently, as the microwave ovens are widely used, inexpensive, small sized, and light magnetrons have been needed. In the cathode structure of the magnetron, as shown in FIGS. 1A and 1B, two leads 25, 26 of expensive molybdenum bars have been used as a support of the cathode structure.
As a result, when the leads are made thin to save the material, the vibration-resistant property and the high-impact property of the cathode will be degraded. Furthermore, in order to increase the mechanical strength, two leads 25, 26 are inserted into the spacer 28, and thus, when the thermal expansion of the leads 25, 26 occurs, the leads 25, 26 and the spacer 28 move under frictional resistance relative to each other, and extraneous sounds have been produced.
In other words, in the prior art structure shown in FIGS. 1B, 1C and 1D, the following drawbacks have been involved.
(1) Since the spacer is permitted to rotate relative to the leads, the degree of freedom of movement of the leads is considerably large.
(2) When the looseness between the spacer hole and the leads is made small, the breakdown strength of the cathode against vibrations is increased, however, the extraneous sounds are easily produced.
(3) The spacer 28 is inclined also in the axial direction about a supporting point, that is, a metal sleeve 29 which is a stopper. Thus, even when the looseness is made large, the frictional resistance between the spacer and the leads is not prevented, and the extraneous sounds are produced.